HPACK Denial of Service vulnerability (HPACK Bomb)
A HTTP/2 implementation built using any version of the Python HPACK library between v1.0.0 and v2.2.0 could be targeted for a denial of service attack, specifically a so-called "HPACK Bomb" attack. This attack occurs when an attacker inserts a header field that is exactly the size of the HPACK dynamic header table into the dynamic header table. The attacker can then send a header block that is simply repeated requests to expand that field in the dynamic table. This can lead to a gigantic compression ratio of 4,096 or better, meaning that 16kB of data can decompress to 64MB of data on the target machine.
Affected software
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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | hpack | >= 1.0.0, < 2.3.0 | 2.3.0 |
References
- https://python-hyper.org/hpack/en/latest/security/CVE-2016-6581.html
- http://www.securityfocus.com/bid/92315
- https://github.com/advisories/GHSA-ffq8-576r-v26g
- https://nvd.nist.gov/vuln/detail/CVE-2016-6581
- https://github.com/pypa/advisory-database/tree/main/vulns/hpack/PYSEC-2017-87.yaml
- https://github.com/python-hyper/hpack
- https://web.archive.org/web/20200227233559/http://www.securityfocus.com/bid/92315
- http://python-hyper.org/projects/hpack/en/stable/security/CVE-2016-6581.html
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H- Weakness (CWE)
-- Platform
- Python (PyPI)
- Package
hpack- Disclosed
- 10.01.2017
- Updated
- 11.10.2026 12:18
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2017-87 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-ffq8-576r-v26g - CC BY 4.0
Vulnerability history: hpack
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 2
- First → latest
- 01.2017 09.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- High 1
- Medium 1
Disclosures per year
- 2017: 1
- 2018: 0
- 2019: 0
- 2020: 0
- 2021: 0
- 2022: 0
- 2023: 0
- 2024: 0
- 2025: 0
- 2026: 1
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HPACK Denial of Service vulnerability (HPACK Bomb) this entry
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